1. Field of the Invention
The present invention relates to a tape cover of a magnetic tape cassette and a preparation thereof.
2. Description of the Prior Art
The conventional tape cassette or cartridge for a video tape cassette comprises a casing (A) holding a magnetic tape and a tape cover (B), provided in front of the casing, for protecting the magnetic tape-running area from dust or mechanical damage as shown in FIG. 1 and the tape cover (B) is hinged to the casing (A) at hinged parts (C),(C') of both ends. Symbols D, E designate the central holes of hubs for winding the magnetic tape.
FIG. 2 is a partially enlarged schematic view of the tape cover (B) and FIG. 3 is a sectional view taken along line A--A'. The tape cover comprises a front plate (1), side plates (2),(3) and an upper plate (4). A reverse insertion detecting notch (5) for preventing the insertion of the cassette or cartridge into a cassette set of the video recorder in the erroneous direction, is formed in the boundary of the front plate (1) and the upper plate (4). When the tape cover (B) is formed, the resin-injection port of a mold is provided at a position on the lower edge (6) of the front plate (1); a resin is injected from the port and the gate or sprue formed in the port is cut off after the injected resin is cured. However, such a treatment after the molding requires excessive time for treating the runner in the molding operation; for removing the runner; for grinding and for transportation required for these operations and decreases productivity, thus increasing the total molding and treating time.
The inventors have studied to overcome the disadvantage described above. As a result, they have found that a runner-less product is required to reduce the number of process and the position of the sprue is limited because the entire surface of the molded product must not be treated as it is already a final product.
The inventors have found that the most preferable position is the point designated by the reference numeral (7) in the reverse insertion detecting notch area. However, when the sprue is formed at this point, the following disadvantage causes decrease in the value of the product. The disadvantage will be explained referring to FIG. 4 which is a partially enlarged view of a part of the mold for forming the notch (5) shown in FIGS. 2 and 3 and is a left side view of FIG. 2.
The reference numeral (10) designates a stationary mold and (18) designates a movable mold. When both molds are closed, a cavity (11) for the tape cover is formed. The reference numeral (4) designates the upper plate of the tape cover formed when the resin is injected and (5) designates a portion for forming the reverse insertion detecting notch.
A passage (17) for injecting molten resin is provided in the stationary mold (10) and a heating element (8) extending to a gate portion (13) at the lower end of a body (9) for melting the resin is positioned in the passage (17). Heat energy is given to the heating element (8) so as to heat the resin (12) remaining near the gate portion (13) at the same time as resin is injected by the molding machine.
In the operation, the molten resin (12) remains in the gate portion (13) in a semi-molten state in the non-operation time between the injecting times. When the resin is injected, it is melted by heat from the heating element (8) and is injected through the outlet of the gate portion (13) into the cavity (11) of the mold. However, a portion of resin (14) remaining in the gate portion (13) flows into the cavity (11) in a highly viscous state whereby it gathers in a position (15). The continuously injected resin follows to paths shown by arrow lines (16) to flow into each part of the cavity (11). A flow mark is caused in the molded product by the viscous mass at the position (15) so that the appearance of the surface of the upper plate (4) is poor.